This invention relates to photodetectors and, more particularly, to InGaAs photodiodes.
Lightwave communications systems in the long wavelength range of about 1.0-1.6 .mu.m have stimulated the development of photodiodes from In.sub.0.53 Ga.sub.0.47 As because the bandgap (0.75 eV) of this material is suitable for efficiently detecting lightwaves in that wavelength range and because the material is essentially lattice matched to InP substrates.
These devices may be either mesa photodiodes or planar photodiodes. The former tend to have lower dark currents but generally have not been adequately passivated and have low reliability for many important systems applications. Planar InGaAs photodiodes, on the other hand, trade off somewhat higher dark currents for better passivation and higher reliability.
In addition, InGaAs photodiodes may be either front-illuminated (lightwaves incident on the epitaxial layers) or back-illuminated (lightwaves incident on the transparent InP substrate). The front-illuminated variety is described by Y. Tashiro et al in an abstract entitled "InGaAs Planar Photodiode with a Window Layer," 1981 Domestic Meeting of the Semiconductors and Materials Division, the Institute of Electronics and Communication Engineers of Japan. FIG. 1 of the abstract shows a photodiode comprising an n-InP substrate, and InP buffer layer, an n-InGaAs light-absorbing layer, an n-InGaAsP window layer and p.sup.+ -zone formed in the latter two layers by Cd diffusion. The p.sup.+ -zone is formed under the opening of an annular contact on the window layer. Lightwaves directed through the opening are incident on the window layer and are absorbed in the InGaAs layer. The purpose of the InGaAsP window layer is to provide "good quantum efficiency" (about 80 percent in the wavelength range 1.3-1.6 .mu.m as shown in FIG. 3 of the abstract). A dark current of about 1 nA at -5 V bias at room temperature is suggested by FIG. 2 of the abstract. In contrast, when a back-illuminated configuration is used, lightwaves are made incident on the transparent InP substrate, and so the InGaAsP window layer is omitted: that is, because lightwaves would not be incident on the window layer, it would have no affect on the quantum efficiency of the device. One such device is described by M. El Hamamsy et al in copending application Ser. No. 359,988 filed on Mar. 19, 1982, now abandoned. As with the front-illuminated photodiode, this back-illuminated device includes an n-InP substrate, an n-InP buffer layer, and an n-InGaAs light-absorbing layer, but contains no InGaAsP window layer. This InGaAs photodiode achieves dark currents of about 10 nA at -10 V bias by means of a restricted contact configuration shown in FIG. 1 herein. In addition, it is well passivated and highly reliable.
However, even lower dark currents would increase the photodiode's sensitivity and thus enable optical receivers to operate better at lower bit rates and/or higher ambient temperatures.